MANURES 41 



an *' available " form, that is to say, available 

 to the plant, which they can only be if they are 

 soluble in water or in the acid sap which plant 

 roots exude, as plants can only absorb fluids 

 and not solids. For example, it is possible for 

 a soil to be rich in potash and yet require 

 applications of a manure supplying potash ; this 

 is because the potash in the soil is in an 

 "unavailable" state, that is to say, it exists in 

 the form of a chemical compound which will 

 not dissolve, and so is useless to the plant. 



Nitrates require additional notice as, owing 

 to their being readily soluble, they are easily 

 lost in the drainage waters leaving land if it 

 is not under a crop. Further, under certain 

 conditions they are converted to the gas 

 ammonia, and this passes away into the air and 

 so is lost. 



It is not intended here to give details of the 

 different manures available, as there is already 

 ample literature to be had concerning them, but 

 a classification of them is necessary. 



All manures fall into one or other of the 

 following classes : — 



(i) Nitrogenous manures, which supply 

 nitrates only, are nitrate of soda and sulphate 



